Embodiments of the present invention relate to a deposition process for filling recesses in a substrate.
Electronic circuitry, such as integrated circuits, is formed by the deposition, formation (such as by oxidation, nitridation, etc.) and partial removal of layers on a substrate. The substrate may be a silicon wafer (although other materials such as gallium arsenide or glass can be used) onto which various layers may be deposited, removed, or partially removed. A substrate can be a single material but is more often a base material having situated thereon one more conducting, insulating (dielectric) or semiconducting materials.
The partial removal of a material on a substrate via reactive gases, commonly known as etching, begins with a step in which a patterned photoresist or mask layer is formed on the substrate by a conventional photolithographic method. In the following etching step, the substrate is placed in a chamber and exposed to an energized plasma of a gas that is energized by, for example, microwave energy or radio frequency energy. A biasing electrical voltage may be coupled to the energized gas so that charged species (reactive ions) within the gas are energized toward the substrate. In the etching method, recesses shaped as narrow channels, holes, or trenches, are formed in the substrate.
After etching, deposition methods are used to fill the etched recesses, for example, with a dielectric or conducting material. However, in some conventional recess-filling methods, voids are often formed within the material being deposited in the recess during the recess-filling process. The voids are especially prone to being formed when the sidewalls of the recesses are not smooth which arise from the narrow widths of the recesses. In the deposition process, overhangs that form on the non-smooth recess sidewalls often coalesce to form a void in the recess.
In a “bottom-up” deposition method, the recesses are attempted to be filled without voids, by maintaining a higher deposition rate at the bottom of the recess relative to the deposition rate on the sidewalls of the recesses. For example, when silicon oxide is deposited in a recess, the deposition rate of the silicon oxide on underlying silicon material is higher than the deposition rate of the silicon oxide on underlying silicon nitride material, as for example, shown in FIG. 2. The higher oxide deposition rates on silicon reduce the formation of overhangs during filling of the recess, especially when the silicon material is at the bottom of the recess and the nitride material is on the sidewalls of the recess. For example, the bottom-up deposition method is often applied in shallow trench isolation processes. In such methods, a precursor deposition gas, such as a combination of O3 (ozone) and TEOS (tetraethylorthosilicate) having a high ratio of O3 to TEOS is used to enhance the selectivity of the deposition growth rates for both nitride and thermal oxide. The higher the ratio of O3 to TEOS the higher is the deposition rate selectivity ratio, which in turn, affects the recess-filling capability.
However, when the bottom-up method is used to fill recesses 27 in a substrate 20, some recesses 27 are easily filled but others are not, even when the recesses 27 are not very narrow, as for example, illustrated in FIG. 1. In this substrate 20, a plurality of recesses 27, such as trenches, are etched between polysilicon gates 22 on the substrate 20. The sidewalls of the polysilicon gates 22 are covered with nitride spacers 24. A conformal nitride liner 26 covers the nitride spacers 24, polysilicon gates 22, and other portions of the substrate, such as the bottom of the recesses 27. After an oxide layer 28 is formed to fill the recesses 27, some of the recesses 27 are not easily filled, and voids 30 are still formed in these recesses 27.
Generally, the recesses 27 which are not easily filled without forming voids 30 are those which have non-smooth profiles with some reentrant cavities 10 thereon, as shown in FIG. 3. To find a method for filling these recesses 27, precursor gas compositions having different O3 concentrations, and different process temperatures were tried. Although using different process parameters was determined to improve recess filling a little, the voids 30 were still formed in some of the recesses 27 because of their non-smooth profiles. For example, voids 30 were formed even in the small tapered recesses 27 when the sidewalls of the tapered recesses 27 had the reentrant cavities 10.
Thus, it is desirable to have a deposition method that is capable of filling recesses, such as trenches, in a substrate, without forming voids in the recesses.